Ultrasonic distance amplitude correction unit

ABSTRACT

An automatic distance amplitude correction device and circuit which automatically corrects for amplitude variations in signals such as caused by the attenuation of sound propagating through a test specimen. A signal is transmitted through a test specimen and the reflection is detected and used to control the time constant of an integrating circuit so that the signal applied to a compensating network for controlling the gain in the receiver is automatically adjusted as a function of the amplitude of the received signals.

13-a31 SR GR 3 s 69 D s 1 3 *5 Ma 1 United St: [151 3,690,153

Matay 1 1 Sept. 12, 1972 [54] ULTRASONIC DISTANCE AMPLITUDE 3,090,2245/1963 Rankin ..73/67.9 CORRECTION UNIT 3,287,962 1 1/1966 Relyea et a1...73/67.9

[72] Inventor: Matay North Royalton Primary Examiner-Richard C. QueisserAssistant Examiner-Arthur E. Korkosz Asslgnee! TRW Cleveland, 01110Attorney-Hill, Sherman, Meroni, Gross & Simpson [22] Filed: Dec. 7, 1970[57] ABSTRACT [21] Appl. No.: 95,538

An automatic distance amplitude correction device and circuit whichautomatically corrects for amplitude :Jf-il. variations in Signals Suchas caused by the attenuation 1 ll of Sound p p g g through a testSpecimen A Fleld Of Search Signal is transmitted through a test p i andthe 3 0/3 reflection is detected and used to control the time constantof an integrating circuit so that the signal ap- [56] References Cltedplied to a compensating network for controlling the UNITED STATESPATENTS gain in the receiver is automatically adjusted as a function ofthe amplitude of the received signals. 2,583,531 l/1952 Hathway ..340/3R 3,048,031 8/1962 Beajard ..73/67.8 4 Claims, 9 Drawing Figures 47k4/V50(64 I 77/145 PULjEK //Z k a? /7 m Q Rt'f/l/-K 29 6/1750 wan: 4MP

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57 6/776 Para/mu. 1/ 1/ v 13 /4 MMATAY ULTRASONIC DISTANCE AMPLITUDECORRECTION UNIT BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates in general to ultrasonic nondestructive testequipment and in particular for automatic means for controlling thereceiver sensitivity so as to adjust for the attenuation of soundpropagation through a test specimen.

2. Description of the Prior Art Ultrasonic non-destructive testequipment has provided for controlling the sensitivity of the receiverin which a time constant circuit has been utilized to provide anexponential decay of an applied voltage which is used to vary thesensitivity of the receiver. The time constant circuit has beenstatically adjusted in the past as for example by varying the value ofcapacitance or resistance and the same time constant has been used fortesting as long as the manual adjustment of the resistance andcapacitance are not changed.

SUMMARY OF THE INVENTION The present invention comprises an improvementin ultrasonic test equipment in that an automatic correction for thevariable response characteristics in the ultrasonic tests which arecaused by material structure is obtained. The effects of variableattenuation in the test specimen are compensated by detecting a signalfrom the test specimen whose response is proportional to the specimensattenuation and by controlling the time constant automatically of anexponential circuit in response to the amplitude of the reflected ortransmitted signal. Thus the exponential circuit provides for acontinuously varying time constant dependent on the amplitude ofreceived signals and the output of this circuit is utilized to vary thesensitivity of the receiver to provide a uniform signal independent ofthe thickness and attenuation of the specimen. Since the output of thereceiver is presented on a time base, the reflections from variousdiscontinuities in the specimen will be more realistically presented toan observer and the variations in amplitude of signals caused by varyingdistances and attenuations in the specimen will be compensated.

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof taken in conjunction with the accompanying drawings althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates test equipment withthe invention incorporated therein:

FIG. 2 is a plot of gate potential versus channel resistance of a fieldeffect transistor;

FIGS. 3A-3D illustrate wave shapes appearing at different points in theinvention;

FIG. 4 illustrates a modification of variable time constant circuit;

FIG. 5 illustrates the voltage versus capacitance characteristic of thecircuit of FIG. 4; and

FIG. 6 is a schematic view of the compensating network illustrated inFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates in blockform an ultrasonic test equipment for testing the characteristics of aspecimen 10. A pulser 14 periodically applies to the specimen 10 anultrasonic signal through the transducer 13. The signal passes then tothe specimen at the front interface 11, passes through the specimen andis reflected from the rear interface 12. The reflected signal passesthrough the transducer 13 and to a receiver 17 which applies an outputon a scope 18. A timer 16 supplies an input to the pulser 14 so that itperiodically transmits a signal to the specimen and also supplies aninput to a multivibrator 19 which is connected to a switch 21 that maybe controlled by the knob 22. In a first position of the switch 21 itengages contact 23 to apply the output of the multivibrator 19 to amanual slope generator 26. The output of the manual slope generator 26is connected to a compensating network 27 that applies an input to thereceiver 17 through the lead 28 which controls the amplitude sensitivityof the receiver 17 as a function of distance through the specimen 10.The ultrasonic test equipment also provides a gated video amplifier 29which receives an input from the output of the receiver 17 and appliesan output to a recorder amplifier 31 which supplies an output to arecorder 32 for making a permanent record of the response of thespecimen.

The improvement of the present invention utilizes the output of theamplifier 31 applied through a diode D1 to resistor 33 which has itsother side grounded. A slide contact 34 is controllable by a knob 36 andapplies an input to the gate of a field effect transistor 38. The sourceof the field effect transistor is connected to ground and the drain isconnected to contact 39. A capacitor C1 is connected between contact 39and contact 24 which may be connected to movable contact of switch 21 toreceive the output of the multivibrator 19. A compensating network 41receives a signal from contact 39 and applies an input to the receiver17 through lead 42.

When the switch 21 engages the contact 23 to apply the output of themultivibrator 19 to the manual slope generator 26, the apparatus of FIG.1 operates according to the prior art. The multivibrator 19 is keyed bythe timer l6 and produces pulses 48 illustrated in FIG. 3C which areapplied to the manual slope generator 26 that might comprise an RCcircuit for producing an exponential decay of the pulse 48 and which isapplied to the compensating network 27 which in turn controls the gainof the receiver 17 through the lead 28. The manual slope generator 26according to the prior art has a fixed characteristic for a givenattenuation value and once set produces a fixed output wave shape andthe compensating network 27 provides a fixed sensitivity control outputto the receiver 17 as a function of distance. These prior art devicesadjust the beam energy intensity characteristics as a function of soundtravel distance from the transducer 13 and as a function of distance forthe attenuation characteristics due to the material structure of thespecimen 10. The slope generator 26 is set for a particular transducerand for a particular amount of attenuation. However, where theattenuation coefficient of materials vary greatly, the automaticdistance amplitude correction unit of the present invention is desirablewhich provides an automatic correction for the variable responsecharacteristics in the specimen under test which are caused by thematerials structure.

Variations in sound intensity result whenever the material in the travelpath of the ultrasonic beam reflects or absorbs more or less of theultrasonic energy. The reflected energy in the ultrasonic wave frontthat is returned to the transducer 13 in pulse echo tests is relatableto the dimension of a defect. If the amount of energy reaching a defectvaries, the reflected energy from such defect will also vary.

The automatic distance amplitude correction unit according to thisinvention provides for a more consistent display of images of defectswhich are related to the defect size by correcting for variableattenuation through the medium.

When the switch 21 in FIG. 1 engages contact 24 so as to connect thevariable time constant circuit 37 into the system, automaticdistance-amplitude correction will occur.

The operation of this circuit will be explained with reference to thewave shapes illustrated in FIGS. 3A-3D. The intensity of any receivedsignal from within the material is a function of the thickness x of thematerial and the attenuation coefficient a for a given material and thesound energy transmitted into the specimen.

Where I is the intensity of the received signal, I, is the transmittedsignal.

In thick materials the attenuation becomes significant and must becorrected for by varying the gain of the receiver as a function ofdistance (time). The intensity of the ultrasonic waves in the materialdecreases exponentially in accordance with the above formula as afunction of both attenuation and thickness. The amplifier sensitivity iscontrolled as a function of time (distance) to provide a uniform signalregardless of the distance (thickness) and attenuation relationships.The prior art correction unit is illustrated with the circuit whenswitch 21 is engaged with contact 23 but is subject to the disadvantagethat the correction assumes a constant attenuation which is fixed anddepends on the particular characteristics of the manual slope generator26 and the compensating network 27. Variations in intensity caused bychanges in attenuation are ignored. In the present invention, however,there is an automatic correction for attenuation variations illustratedwith the switch 21 in engagement with contact 24, the wave shapeillustrated in FIG. 3D is applied to the ultrasonic receiver 17 throughthe compensating network 41 in order to control its sensitivity toachieve a constant intensity I. This control signal can be generated bymonitoring the pulse amplitude reflected from the rear interface of thetest specimen, the pulse amplitude of the noise generated due toreflections from the specimens structure or the pulse amplitude of athrough transmitted signal. It is to be noted that as shown in FIG. 3Dthe exponential control signal varies between T and T The wave shapeillustrated in FIG. 3D is established by an RC network comprising thefixed capacitor C1 and a variable resistor comprising the drain tosource impedance of the field effect transistor 38. The field effecttransistor 38 has its source connected to ground and its drain connectedto the capacitor Cl. By varying the potential on the gate of the fieldeffect transistor 38, its drain to source impedance may be varied asillustrated by curve 43 in FIG. 2. For example, FIG. 2 illustrates thechannel resistance versus gate potential of a field effect transistorthat might be a type HEP-801 which might vary between 0.4 to k0 as thegate potential is varied respectively from 0.1 to 1.5 volts. Thus byvarying the gate potential of the FET 38 the time constant of thecircuit 37 can be adjusted.

The time constant circuit 37 differentiates the trailing slope of thesquare wave output of the multivibrator 19 comprising the pulses 48illustrated in FIG. 3C. The field effect transistor has a square lawcharacteristic and its channel resistance between its drain to sourcemay be varied over a wide range. The control of the resistance fromdrain to source of the field effect transistor is made to beproportional to the attenuation of material under test. The signalproportional to the attenuation of the material under test is obtainedfrom the recorder amplifier 311 through the diode D1 and is applied tothe resistor 33 and supplied from the variable tap 34 to the gate of thefield effect transistor 38. The diode D1 comprises a feedback uncouplingmeans. The automatic distance amplitude correction circuit operates asfollows. The variable attenuation of the material will cause theemergent signal to vary between the values of Imat and 1, illustrated in3A bypulses, 47. The gated video signal passing to the amplifier 31 willvary between Vcsmar and VGSmm illustrated in FIG. 38 by wave shapes 45.This signal will be applied to the resistor 33 and the contact 34 to thegate of the field transistor 38 to obtain various channel resistances asillustrated in FIG. 2. Thus the time constant of the network 37 willvary between the values of T and T as shown in FIG. 3D. The outputsignal appearing at terminal 39 will be applied to control the gain ofthe receiver 17 through the compensating network 41 to automaticallycorrect for distance amplitude variations.

FIG. 6 is a schematic of a compensating network 41 utilized in an actualembodiment of the invention. The output terminal 39 of the variable timeconstant circuit 37 is connected to the base of a transistor T1 formingpart of the compensating network 41. The emitter of the transistor T1 isconnected to ground through resistor R2. The collector of transistor T1is connected to a suitable bias source through a resistor R1. Couplingcapacitor C2 couples an output signal to a direct current level shiftercomprising the transistors T2 and T3 in order to raise the controlsignal by R3 to the required bias level of the ultrasonic RF amplifiertube in the receiver 17. The collector of transistor T2 is connected toa suitable bias source and the emitter is connected to a resistor R3which has its other side connected to the collector of transistor T3.The emitter of transistor T3 is connected to a bias source throughresistor R4 and the base is connected to the bias source through aresistor R5. The base is also connected to ground through a resistor R6.

A variable contact 60 engages the resistor R3 and is connected to thebase of an emitter-follower transistor T4. Capacitor C3 is connectedbetween ground and the base of transistor T4. A zener diode D2 isconnected between a bias source and the collector of transistor T4 and aresistor R7 is connected between the emitter of transistor T4 andground. The output of the compensating circuit 41 is applied to lead 42and is applied through a switch 61 to the grid of an amplifier tube V1which might be in the third RF amplifier stage of the receiver 17, forexample. In a typical commercial unit a capacitor C6 is connectedbetween ground and lead 42 and inductors L1, L2 and L3 are connected inseries between the grid of the tube VI and the capacitor C6. The switch62 is connected to selectively short out the inductors L2 and L3. Aresistor R8 is connected between the cathode of tube V1 and ground. Theplate of tube V1 is connected to terminal 62.

In an actual circuit the component values of elements in FIG. 6 were asfollows:

R1 5.9 R9 R6 2 m [.8 R R7 200 0. R3 1 R0 R8 200 0 R4 1 m 02 0.05 ,t F Rs680 0 ca 1500 p F C6 0.02 ,LF

Transistors T1, T2 and T3 were type NPN 2N585; transistor T4 was typePNP 2N404; Zener diode D2, type 1N752; -l2 Volts bias was applied to thediode D2 and to the resistor R4; and, +12 Volts bias was applied to thecollector of transistor T2 and to the resistor R1.

The circuit of FIG. 6 allows the automatic distance amplitude outputsignal to be inverted so that it is nega tive and such that it may beapplied to the grid of the tube VI of the RF amplifier. It is to benoted that the signal appearing at terminal 39 is a positive signal andthat this signal could be used directly to control the gain of the RFamplifier. It would be applied to the cathode of the RF amplifier stage.However, the use of the circuit of FIG. 6 allows this signal to beinverted so that negative signal may be applied to the grid of theamplifier stage V1. Although the compensating circuit illustrated inFIG. 6 is shown as connected to only a single radio frequency amplifierstage in the receiver 17, it is to be realized that the automaticdistance amplitude correction signal may be applied to all of the RFamplifier stages at appropriate rates. Also, in order to achieve smoothcontrol the amplifier tubes are selected so that they do not operate inthe vicinity of cut-off.

A modification of the invention is illustrated in FIG. 4 wherein thevariable time constant circuit 37 comprises a fixed resistor R9connected in circuit with voltage variable capacitors C8 and C9. Asshown in FIG. 4, the output from multivibrator 19 may be applied throughterminal 24 to voltage variable capacitor C8 which is connected inseries with a second voltage variable capacitor C9 to output terminal39. Resistor R9 is connected between the output terminal 39 and ground.The control voltage is applied from wiper contact 34 through an inductorL4 to the junction point between the voltage variable capacitors C8 andC9. The total capacitance of the voltage variable capacitors C8 and C9varies as shown in FIG. as a function of the voltage applied to wipercontact 34. The voltage variable capacitors voltage dependent P/Njunction capacitance C, is controlled by its reverse-bias controlvoltage illustrated in FIG. 3B which is in turn proportional to theattenuation of material under test. The automatic distance amplitudecorrection operation for a voltage variable capacitor is as follows: Thematerials variable attenuation will cause the emergent signal to varybetween the values of I and I,,,,,, and the gated echo level controlledrecorder signal of the ultrasonic units gate will also vary betweenVGSMI and V as shown in detail in FIG. 3B. This signal will cause thevoltage variable capacitors P/N junction capacitance, C,, to vary inaccordance with its characteristic curve illustrated in FIG. 5. FIG. 5is a voltage variable capacitor characteristic for a type TIV303 TexasInstruments Capacitor.

Thus, by varying the voltage at wiper 34 the time constant of thecircuit illustrated in FIG. 4 will vary to change the characteristic ofthe output signal at terminal 39. This signal is applied as a biassignal to the receiver to maintain the receiver output constant.Although a voltage variable capacitor is illustrated, it is to berealized that such devices are also referred to as varactors," varactordiodes," and varicaps" in the electronic industry.

Two voltage variable capacitors are used so as to isolate the controlvoltage appearing at terminal 34 from the input square wave signalapplied at terminal 24. The inductor L4 is utilized to avoid possiblefeedback effects.

It is seen that this invention provides an automatic distance amplitudecorrection circuit for automatically compensating for attenuation ofsignals due to sound wave attenuation and travel distance.

Although the invention has been described with respect to preferredembodiments it is not to be so limited as changes and modifications maybe made therein which are within the full intent and scope as defined bythe appended claims.

Iclaim:

1. An automatic distance amplitude correcting device for a receiver fortesting a specimen comprising:

means for periodically transmitting a signal through said specimen;

said receiver receiving said signal from said specimen;

an automatic gain control element in said receiver;

a multivibrator connected to said means for periodically transmitting asignal;

an automatically variable time constant circuit receiving an output ofsaid receiver and the output of said multivibrator; and

a compensating network receiving the output of said automaticallyvariable time constant circuit and supplying an output to said automaticgain control element in said receiver which comprises a voltage whichexponentially decays at a rate depending on the output of saidautomatically variable time constant circuit.

2. An automatic distance amplitude circuit according to claim 1 whereinsaid automatically variable time constant circuit comprises:

a capacitor with one side connected to the output of said multivibrator;

a field effect transistor with its drain connected to the other side ofsaid capacitor and to the input of said compensating network; and

the gate of said field effect transistor connected to the output of saidreceiver to vary the impedance across the field effect transistor as afunction of the output of said receiver.

ries between the input of said multivibrator and said compensatingnetwork;

a resistor connected between ground and the input of said multivibrator;and

an output of said receiver connected to the junction point between saidpair of voltage variable capacitors.

1. An automatic diStance amplitude correcting device for a receiver fortesting a specimen comprising: means for periodically transmitting asignal through said specimen; said receiver receiving said signal fromsaid specimen; an automatic gain control element in said receiver; amultivibrator connected to said means for periodically transmitting asignal; an automatically variable time constant circuit receiving anoutput of said receiver and the output of said multivibrator; and acompensating network receiving the output of said automatically variabletime constant circuit and supplying an output to said automatic gaincontrol element in said receiver which comprises a voltage whichexponentially decays at a rate depending on the output of saidautomatically variable time constant circuit.
 2. An automatic distanceamplitude circuit according to claim 1 wherein said automaticallyvariable time constant circuit comprises: a capacitor with one sideconnected to the output of said multivibrator; a field effect transistorwith its drain connected to the other side of said capacitor and to theinput of said compensating network; and the gate of said field effecttransistor connected to the output of said receiver to vary theimpedance across the field effect transistor as a function of the outputof said receiver.
 3. An automatic distance amplitude circuit accordingto claim 2 including a resistor with a slide contact connected betweenthe gate of said field effect transistor and the output of saidreceiver.
 4. An automatic distance amplitude circuit according to claim1 wherein said variable time constant circuit comprises: a pair ofvoltage variable capacitors connected in series between the input ofsaid multivibrator and said compensating network; a resistor connectedbetween ground and the input of said multivibrator; and an output ofsaid receiver connected to the junction point between said pair ofvoltage variable capacitors.